1. Field of the Invention
The present invention relates to an improvement of austenite stainless steel having high crevice corrosion resistance and hot workability, and advantageously used for various parts, for example parts used in seawater or a flue gas desulfurization apparatus.
2. Description of the Prior Art
Stainless steel has high corrosion resistance, and is used in various industrial fields. However, when stainless steel is used in an atmosphere of high chloride ion, for example used in seawater or a flue gas desulfurization apparatus, serious corrosion such as crevice corrosion tends to appear, so that a wide used stainless steel such as SUS 304 and SUS 316 and the like is restricted to use. Therefore, some trials for improvement of corrosion resistance have been made by increasing content of Cr and/or Mo, and/or adding N. For example, Japanese Laid Open No. 1977-95524 typically discloses an austenite stainless steel including more than 6.0% by weight of Mo. However, when content of Cr or Mo is much, intermetallic compound such as .sigma. phase and .chi. phase and the like tends to precipitate in casting which is one of process of manufacturing stainless steel. As a result, lack of Cr or Mo partially appears in a matrix of the stainless steel, this result in decrease of corrosion resistance and hot workability. The decrease of hot workability appears in a way that intermetallic compound does not disappear during heating for hot rolling, so that an end of a hot rolled plate breaks into two in the direction of thickness thereof (hereinafter, call it "breaking into two").
In order to avoid precipitate of intermetallic compound such as a phase and the like, for example Japanese Patent Publication No. 1982-28740 proposes to increase amount of addition of N. However, increase of content of N result in increase of deformation resistance in hot, so that hot rolling can not be carried out in some cases. Therefore, as disclosed in for example Japanese Laid Open No. 1987-192530, it is proposed to introduce a soaking process before and after a hot rolling so as to change precipitate such as intermetallic compound into a matrix which does not influence too much upon the quality of the material and corrosion resistance even if the chemical composition of the alloy is easy to precipitates intermetallic compound such as .sigma. phase and the like. However, introduction of soaking process result in increase of manufacturing cost, this may be a serious obstacle for practical use.